Stereotaxy is a method of precisely locating targets in 3-dimensional space using a reference frame. In stereotactic surgery a stereotactic frame is used to determine the coordinates of the target, and then to guide the approach of surgical instruments to that target using a route which results in minimum damage to surrounding tissues.
In theory, any organ system inside the body can be subjected to stereotactic surgery. There are however difficulties in establishing a reliable frame of reference, such as bone landmarks which bear a constant spatial relation to soft tissues. This means that its applications have so far been limited to brain surgery.
In neurosurgery, stereotactic methods are routinely used in operations to aspirate brain cysts, biopsy tumours or delineate the path to deep seated lesions. The main advantage of stereotactic neurosurgery to ‘open’ brain surgery is the high precision of lesion localisation. The accuracy of target localisation allows operations on lesions deep within the brain to be performed through very small openings. More recently, stereotactic methods have been used to implant electrodes into brain targets. An electrical current is passed through the electrodes to modify brain function and this forms the basis of deep brain stimulation (DBS). DBS is now an accepted neurosurgical treatment used for the management of Parkinson's disease, tremor, dystonia, pain and epilepsy. The potential demand for DBS is great and the numbers of patients undergoing DBS would be even greater if DBS was used for psychiatric and pain procedures and stroke.
One of the major factors determining the success of DBS is the accuracy of electrode placement. This is, in turn, reliant on the accuracy of the method used to determine the target coordinates. The targets are often not much bigger than a grain of rice and in many instances cannot be seen on brain imaging. In these cases the location of the target is calculated relative to a virtual line connecting the anterior and posterior commissure of the brain, the AC-PC line.
The surgery uses a mechanical device called a stereotactic frame that has head-holding clamps and bars which puts the head in a fixed position in reference to the coordinate system (the so-called zero or origin). The stereotactic apparatus uses a set of three coordinates (x, y and z) in an orthogonal frame of reference (Cartesian coordinates), or, alternatively, a polar coordinates system, also with three coordinates: angle, depth and antero-posterior location. It is therefore imperative that the reference frame, i.e. stereotactic frame base upon which the target coordinates are determined, is parallel to the AC-PC line. The AC-PC line runs from the superior surface of the anterior commissure in the brain to the centre of the posterior commissure (Talairach atlas).
There are several different types of stereotactic frames available but the two most commonly employed modern stereotactic frames are the Leksell and the CRW frames from Radionics which are based on the centre-of-sphere concept. These frames comprise of a frame base (or head ring) designed to surround the head in the transverse plane parallel to the AC-PC line. They are made of non ferrous materials such as aluminium so that they can be used with CT and MRI scanners.
A range of attachments are then connected to the frame base, such as head posts, cross bars, arcs and plates. The frame is connected to the patient using self tapping fixing screws that engage with the head posts and are screwed in to the skull. After the frame's fixation, images are taken for preoperative planning. Once localization is completed, the system is prepared for surgery by simply attaching an arc to the frame. The arc is positioned according to the previously calculated X, Y and Z coordinates of the target so its center coincides with the selected cerebral target.
High accuracy of electrode placement during procedures such as DBS is required for the following reasons:                The target tissue volume is small. If the electrodes are not accurately placed the outcome of surgery is poor.        In many instances the target is surrounded by other important brain structures. Accurate electrode placement reduces the risk of complications either as a result of electrode placement or due to electrical spread to surrounding structures.        The DBS procedure is very costly and can take up to 8 hours. A method which could increase the accuracy of electrode placement will not only be clinically beneficial but would have financial implications as well.        
The initial part of deep brain stimulation surgery involves the attachment of the stereotactic frame to the skull parallel to the AC-PC line. The stereotactic frame is first placed around the head and the base (head ring) of the stereotactic frame is aligned with the AC-PC line. Four screws are then passed through head posts attached to the frame base and these screws are attached to the skull under local anaesthetic.
Various methods have been used to align the frame including the use of plastic bars attaching the frame to the ear canals, connecting the frame base across the top of the head with extensible tape or using several people to help align the frame by visual inspection. These methods have the following shortcomings:                They are uncomfortable to the patient.        They do not readily allow fine adjustment to frame position.        They are relatively labour intensive, i.e. requires two or more assistants that are quite likely to be highly paid clinicians.        The process can be very time consuming and if not completed promptly it can delay the surgical procedure, which could lead to a myriad of problems for the patient and operational running of the surgery.        They do not allow easy removal and re-application.        
It is an object of the invention to at least partially address one or more of the above problems.